readme: add contributing section (#1964)
readme: add contributing section (#1964)

Signed-off-by: Matt Klein <684e4e870ef5a243d6d0345280f09d95db603639@lyft.com>
envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,jrajahalme/envoy,fengli79/envoy,envoyproxy/envoy,dnoe/envoy,lizan/envoy,lyft/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,rlazarus/envoy,jrajahalme/envoy,rlazarus/envoy,lizan/envoy,craffert0/envoy,istio/envoy,lizan/envoy,lyft/envoy,sebrandon1/envoy,envoyproxy/envoy,istio/envoy,rlazarus/envoy,jrajahalme/envoy,envoyproxy/envoy,amalgam8/envoy,dnoe/envoy,craffert0/envoy,envoyproxy/envoy-wasm,lyft/envoy,lizan/envoy,jrajahalme/envoy,envoyproxy/envoy,dnoe/envoy,craffert0/envoy,rlazarus/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy,eklitzke/envoy,eklitzke/envoy,envoyproxy/envoy,lizan/envoy,sebrandon1/envoy,sebrandon1/envoy,jrajahalme/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,istio/envoy,dnoe/envoy,dnoe/envoy,istio/envoy,istio/envoy,fengli79/envoy,lizan/envoy,lyft/envoy,fengli79/envoy,craffert0/envoy,sebrandon1/envoy,envoyproxy/envoy,fengli79/envoy,eklitzke/envoy,envoyproxy/envoy,eklitzke/envoy,amalgam8/envoy,sebrandon1/envoy,envoyproxy/envoy,amalgam8/envoy,istio/envoy,amalgam8/envoy